(1) Field of the Invention
The present invention relates to dies for extruding ceramic honeycomb structural bodies (hereinafter referred to as "ceramic honeycomb structural-body extruding dies").
(2) Related Art Statement
The ceramic honeycomb structural bodies are used as catalyst carriers for purifying exhaust gases from internal combustion engines, filters for removing fine particles in exhaust gases, and heat exchangers for exhaust gases, and are produced by an extrusion-shaping process.
However, in order to improve catalyst-purifying performance and filtering performance, there has recently been a demand for enlarging the surface area in the ceramic honeycomb structural bodies. For this purpose, it is necessary that the number of cells per unit sectional area of the honeycomb structural body is increased, the thickness of partition walls is decreased, and dimensional precision is increased.
Therefore, the dies used in the extrusion-shaping process are required to have a decreased channel width, a decreased channel pitch, and higher dimensional precision.
In order to satisfy the above requirements, it is known that excellent dimensional precision extrusion-shaping dies with a channel width of not more than 0.3 mm are obtained by forming extrusion-shaping channels in the dies through plating (Japanese patent Laid-open application No. 55-140,514).
It is also known that when the shaping channels of the die as obtained by the above method are abraded with a ceramic material, desired shaping channels are regenerated by chemically dissolving off the abraded plated layer and plating them again (Japanese patent application Laid-open No. 55-140,515).
As honeycomb structural body-extruding dies, there is known an extrusion die having the structure that ceramic material-retaining portions for temporarily retaining the ceramic material therein are provided between ceramic material-supply holes to which a ceramic material is first fed from an extrusion machine and the lattice-fashioned shaping channels giving the shape of the desired ceramic honeycomb structural body (U.S. Pat. No. 3,038,201).
Further, ceramic material-flowing sections are provided between the ceramic material-supply holes and the shaping channels for uniformly flowing the ceramic material through the die (Japanese patent application Laid-open No. 54-8,661).
Japanese patent application Laid-open No. 55-140,515 relates to a method of regenerating a die as mentioned above. However, this method has a drawback in that since the plated layer of Ni or the like is dissolved off with an acid such as nitric acid, a bonding layer, by which die-constituting members are bonded together with silver solder or the like, in partially or entirely corroded with the acid.
When the bonding layer is entirely corroded, the die is decomposed into, for instance, a member having the supply holes for a ceramic material to be extruded (hereinafter referred to as a ceramic material-supply hole member), a member having extrusion-shaping ceramic material retaining portions (hereinafter referred to as a ceramic material-retaining member) and a member having shaping channels (hereinafter referred to as a shaping channel member). Consequently, the die can no longer be regenerated again.
Even if the bonding layer is partially corroded, uneven portions are formed in the corroded bonding layer, so that the flow of the ceramic material is disturbed by the unevenness. As a result, a strain remains in extrusion-shaped bodies to cause cracks therein during firing.
In the die structure disclosed in U.S. Pat. No. 3,038,201 or Japanese patent application Laid-open No. 54-8,661 in which the dimension of the shaping channels is made small and therefore the flowing of the ceramic material needs to be improved, it is necessary from the standpoint of mechanical working that the shaping channel member, the ceramic material-supply hole member, and a member having the ceramic material-flowing sections provided between the shaping channel member and the ceramic material-supply hole member (hereinafter referred to as "ceramic material-flowing member") and/or the ceramic material retaining member are separately machined, and then bonded together.
In this case, since the bonding area is small, even a small degree of corrosion largely causes the deterioration of the bonding strength. Thus, there is a problem in that the shaping channel member and the ceramic material-staying member are separated to disable the assembling thereof.
Besides the case where, as mentioned above, the plated layer in the shaping channels is dissolved off with acid and the die is regenerated through plating, there is a problem in that the useful life becomes shorter because the die is corroded with the ceramic material during the extrusion-shaping process. Thus, the composition of the ceramic material needs to be selected to prevent corrosion of the die.